A conventional variable gear transfer system varies an amount of change in steering angle of tire wheels (steered wheels) relative to an amount of operation of a steering wheel in a vehicle. This variable gear transfer system has a steering sensor for detecting a steering angle of the steering wheel and an electric actuator for varying the rotation transfer ratio in accordance with the detected steering angle, so that the change amount in the steering angle of the steered wheels relative to the operation amount of the steering wheel is varied. Since the variable gear transfer system thus performs steering control by using the steering angle detected by the steering sensor, accurate control cannot be performed if the steering sensor fails to operate normally. Therefore, it is necessary to detect any failure of the steering sensor.
The variable gear transfer system also has an actuator sensor for detecting a rotation angle of an output shaft of the electric actuator, thereby to perform feedback control by comparing an actual rotation angle of the actuator output shaft with a target rotation angle of the same. It is therefore also necessary to detect any failure of the actuator sensor.
US 2004/0147002A1 (JP 2002-340625A) discloses a failure detection device for detecting failure of a steering sensor. This failure detection device performs failure detection for a steering sensor by moving a sensor unit for detecting rotation of a steering shaft a predetermined distance, which is longer than a unit pitch of detection of the steering shaft. The failure of the steering sensor is detected by checking whether a signal produced by the sensor unit changes while the sensor unit is moved.
If failure in the above steering sensor is a mechanical one, it can be detected easily because the signal produced from the steering sensor does not change in a self-diagnosis operation. If failure is an electrical one, there are some cases in which the signal produced from the steering sensor does not change. This failure detection device may not detect any failure in some cases.
JP 2006-177750 discloses a failure detection device for detecting failure of an actuator sensor. In this device, a resolver is energized by a sine-wave signal, amplitude-modulated in accordance with a rotation angle of a rotor relative to a stator, and produces a sine-wave phase output signal and a cosine-wave phase output signal. The amplitude changes of these output signals are different from each other by π/2 in phase. The amplitudes of the sine-wave phase output signal and the cosine-wave phase output signal are calculated to determine the rotation angle in accordance with the calculated amplitudes. If the square-root of a sum of square of each amplitude continues to be outside a predetermined range, failure is tentatively determined. If this failure further continues, the failure of the actuator sensor is determined finally.
In many instances, a steering power assisting system is mounted in a vehicle together with the variable gear transfer system. The steering power assisting system is for assisting steering power. In this system, an input shaft and an output shaft are coupled by way of a torsion bar, and a torsion bar sensor is provided to detect a difference in rotation angles of the input shaft and the output shaft. The assisting power, which is to be generated by an electric actuator, is determined in accordance with the detected rotation angle difference, so that a target rotation angle of the electric actuator is determined in accordance with the determined assisting power. The electric actuator is feedback-controlled so that an actual actuator rotation angle detected by the actuator sensor converges to the determined target rotation angle. It is also necessary to detect failure in the torsion bar sensor and in the actuator sensor.
JP 2005-43071A discloses a failure detection device for detecting failure of a torsion bar sensor This failure detection device is provided for an electric power steering apparatus, which includes twin resolver type torque sensor having a torsion bar, an input shaft rotation angle sensor and an output shaft rotation angle sensor. Failure of the torsion bar sensor is detected, if an inter-phase electrical shorting is detected in either of the rotation angle sensors. This electrical shorting is determined, if the electric angle calculation result by one of the rotation angle sensors is outside a first predetermined range including 45° for more than a predetermined period and the electric angle calculation result by the other of the rotation angle sensor is also outside a second predetermined range including 45° for more than a predetermined period.
U.S. Pat. No. 7,154,404 (JP 2005-219527A) discloses a failure detection device for detecting failure in a rotation angle sensor, which detects rotation angle of an electric actuator in a steering power assisting system. This steering power assisting system includes a three-phase brushless DC motor, a steering torque detector for detecting steering torque generated by a steering wheel, a resolver for detecting a resolver rotation angle of the brushless DC motor, a rotation angle converter for converting the detected resolver rotation angle into a motor rotation angle corresponding to the number of poles of the brushless DC motor, a rotation direction command unit for determining a rotation direction command for the brushless DC motor, a current calculation unit for calculating a command current for driving the brushless DC motor based on the detected steering torque. The failure detection device is connected to the current calculation unit to communicate with the current calculation unit and checks whether the calculation result of the current calculation unit is in a normal range. The failure detection device receives the motor rotation angle and the rotation direction command and checks the current calculation unit based on the received motor rotation angle and rotation direction command, thereby detecting the failure of the rotation angle sensor of the electric actuator.
The above failure detection devices for the actuator sensor and the torsion bar sensor can detect failure only when the failure continues.